1. Field of the Invention
The present invention relates to an exposure/recording apparatus such as a raster plotter employed for defining wiring patterns of, e.g., PWBs (printed wiring boards) or a precise exposure apparatus for photomechanical process, which apparatus on-off controls exposure beams along supplied dot pattern data to perform main scanning and vertical scanning thereby to print desired patterns on photosensitive material, and more particularly, it relates to a method of and a device for controlling the exposure beams for correcting uneveness in recording on the photosensitive material caused in such an apparatus.
2. Description of the Prior Art
In order to print, for example, PWB wiring patterns as shown in FIG. 1 on photosensitive material, an exposure head is generally moved along the patterns of land.fwdarw.wiring portion.fwdarw.land while the land patterns are flash-exposed through slits of prescribed shapes with wiring portion patterns being continuously exposed through slits of prescribed shapes, or only the land patterns are entirely flash-exposed through slits of prescribed shapes in advance, whereafter wiring portion patterns between the respective lands are continuously exposed through slits of prescribed shapes thereby to obtain desired patterns in the so-called vector drafting system utilizing an automatic drafting machine. As will be appreciated by one of ordinary skill in the art, the term "land" means a part of a conductor pattern which is used for connection to electrical components. Such parts are generally circular, as shown in FIG. 1 hereof. The difference between the methods of forming the patterns depends on difference in data inputted in the automatic drafting machine. According to such systems, the patterns can be smoothly printed in boundaries without uneveness, whereas considerable time is required for the printing process.
Additionally, there is provided a plane or drum scanning system called a raster scanning system, which utilizes, e.g., a raster plotter in order to enable high-speed processing. In order to accurately record images in the raster plotter, it is necessary to flash-expose respective ones of fine pixels equally pitched in both of the main and vertical scanning directions by exposure beams without causing pitch uneveness, and various methods have been proposed to improve pitch accuracy in the main and vertical scanning directions. For example, a conventional method of improving the pitch accuracy in the vertical scanning direction has generally been carried out to improve mechanical accuracy of an exposure head feeding mechanism while improving a driver, or the like, of a feed motor in accoracy and tracking, thereby to reduce absolute feed errors. However, such improvement in accuracy and efficiency of the feeding mechanism is restricted as a matter of course. Another method has been generally carried out to detect feed pitches thereby to control a servo system. However, tracking errors are caused in such a method by delay in a mechanical system.
For such reasons, slight unevenness is inevitably caused in feed pitches in the vertical scanning direction leading to irregular beam intervals, whereby printing spots along the main scanning direction meander on photosensitive material. As a result, recorded printing areas are not constant in width. Particularly in a multi-beam simultaneous exposure system, it is necessary to expose fine spots obtained by dividing a vertical scanning feed pitch by the number of beams while dense images are required in most of the case, and hence printing pitch errors caused by errors in the vertical scanning feed pitches cannot be allowed in comparison with other case. When figures for PWBs are exposed/recorded on a film, recording line width cannot be obtained in intended thickness by such uneven pitches, whereby a large number of PWBs are determined as defective by inspection after manufacturing. It is desirable that the recorded printing areas are obtained in intended thickness, in order to reduce such defectiveness.
In the prior art raster scanning system, further, pattern boundaries intersecting with the main scanning direction cannot be smoothly printed. In general, a scanning/exposure light source of a raster plotter or the like is used for a flash control system for fine circular spots having light intensity distribution approximate to a Gauss distribution and hence, in the pattern boundaries intersecting with the main scanning direction, a number of cavities slightly smaller than the radius of the printing spots are caused every line pitch of the exposure beams as shown in FIG. 2. When the exposure beams are sharply focused and the gamma characteristic of the photosensitive material is in high contrast, exposure is substantially performed as shown in FIG. 2. Simultaneously required in PWB patterns are correctness and smoothness particularly in line widths of vertical and horizontal lines, and hence such uneveness in the boundaries as shown in FIG. 2 cannot be allowed.